SIR testing was established as a method for evaluating electronic assembly materials and processes as one measure of reliability. The objective of SIR testing is drive test coupons to failure to determine at under what conditions failure-inducing electrochemical reactions occur so that these conditions can be avoided during routine assembly.
Electrochemical failure occurs in the presence of electrical potential, moisture (from a humid environment) and ionic residue. If any of these is not present, electrochemical reactions can’t occur. The ionic residue can come from unreacted plating residues, improperly cured solder resist or from flux.
These residues can cause corrosion, metal migration or dendritic growth leading to early failure.
SIR testing is performed with industry standard test coupons etched with patterns designed for the purpose (typically interdigitated combs). The insulation resistance of a test pattern is monitored at intervals as temperature and humidity are varied. Revised tests recommend test coupons complete with components.
There are several standards from organisations such as International Electrochemical Commission (IEC), IPC, International Standards Organisation (ISO) that largely replace the now defunct Bellcore specification. Each defines different humidity, temperature, test duration, voltage bias, measurement frequency and test coupon. It is recommended that technicians refer to the latest version of the IPC-9201 The SIR Handbook for detailed guidance on the subject.
More information on SIR testing is available from Gen3 Systems’ website (www.gen3systems.com).